RFR(XS): 8221833: Readability check in Symbol::is_valid not performed for some addresses

[Doerr, Martin]

Hi,

I'd like to fix a minor bug in Symbol::is_valid which can cause errors during error reporting:
Address computation can overflow leading to skipped readability check.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8221833

Webrev:
http://cr.openjdk.java.net/~mdoerr/8221833_valid_symbol/webrev.00/

Please review.

Best regards,
Martin